Modulation



MarchZO, 1951 E. G. HOPKINS n 2,545,955

MoDULATIoN Filed Jan. 1v, 1948 2 sheets-sheet 1 IEIIII P INVENTOR EDv RD G. HOPKINS ATTO R N EY March 20, 1951 E. G. HOPKINS 2,545,955

Y MoDULATIoN Filed Jan. 17. 1948 '2 sheets-sheet 2 OUTPUT lm/ E-ir-U INVENTOR EDW HQPKINS ATTORNEY Patented Mar. 20, 4 1951 UNITED STATES AFENT F ICE 2,545,955 MoDULA'rI'o Edward Gpodnian Hopkins; Sydney; New' South-y Wales4 Australia@` assignor to Amalamated" Wireless (Australasia) Limited, Sydney,` New Southv Wales; Australia, a company' of New South Wales Australia- Appneatqnranuary 17, 1ers;V seriarb; 21862" In" Australia lebruar'y` 171,' 1947' 9 Claims. gr;

The present invention relates to phase and frequency modulation generally and more par.- ticularly to an electron beam deflection system for producingfa largedegree of such' modulation.

One known method of producing frequency modulation consists of passing` a stabilised carrienv through ai circuit.v which: producesV a phase displacement proportional to the instantaneous value of.4 the modulating` signal. The modulating signalf isv iirst` passed through a. circuit, the attenuation of Wlaicltll is directly proportional to frequency. The particular section of thisknown systeml which produces a phase displacementA in the carrier proportional to' the applied signal voltage is generally referred to as a phase modulator. The requirements for such af device are that it should produce'suiicientphase displacement Without` departure from linearity, to satisfy the requirements. of the system inl whichv itworks. If: in a givenV system the nall carrler frequency and the maximum deviation are specied, an insui'licient variation: in the degree of phase displacement inl the phase modulationv Willset; aA limit` to the loWestmodulating frequency whichY the system canfhandle for a given number of frequency multiplications between the phase modulator and the. final` carrier.

This relationship is expressed by the'follovvingv equation:

Alf A6- Wher A'l'=phas'e deviation Af=frequency deviation fm=modulation frequency When asignal is multiplied, Af ismultiplied by the same amount". By Way of illustration the present standard carriersV for FLM'. broadcasting and mobile operation will be' considered. Fol" broadcasting; the maximum deviation of :E kc; and theminimum'- modulatingfrequency' of" 302 cycles maken ,f or 2,500 raliansV a I-LF. transformer.'

2. comply with tl'ie' requirements of either of tle" sevicesreferred t'o` 'Ilie pri'nci-pal? object dii' the present` invention is to provide a simple means for producing tiail linear' phase deviations4 of sev-erali raclia'ns';4 thusE simplifyingj transmitter' d'esig-nv and greatly reducing'- tlievnumber of necessary multipllcation's;

Vario-us"l methods have been suggested the rerstfo'rI acliiev-ingl thisf objective; but thesei prier systems have involved the use= of such! corn-l plieated circuitl arrangements as to" greatly re-r strictA the* scope oi" their practical applicatiolr.

Som-ef` phase module.tors'V previously` proposed for achieving thef principali ob'ect of the present' invention utilise theA follcu'firlgv principle:A

lai carrier is# divided into two channels' Whosephase` relationshipis degrees, and' these two signals are'K modulated suclr a Way' that the a-rnplitudt` el*l one* is'- proportionaii at any ine stant to the sine of the modulatinav` signal1 and" that of the other is' proportional toits cosine. then the sum of theA two represents a carrier ofconstant amplitude whose phase displacement` from the'- original carrier is directly?proportionalV to the instantaneous valueJ of themodulating signal. A discussion on thisprincipl'e is .riveli` Icy'v Samuel? Sabardl in' Communications oi- September 1941. As' the present invention is` basedo'nthe sane=ge'neral principle, thev previous scheme' will' be' considered i'nbrief.

The requirernerits"A for such a systexir` are` as' follows: y

1. A circuit-Whicr divides the carrier ntotw'd channels', the phaseA difference between which is 90 degrees.`

2". I-vvo` balanced modulators; one for eacl channel.

3. A system which is placed between the modulating signal and eaclr modulator, suchf that? the voltage applied' to onef modulatorwill followthe sine of the modulatigvoltage" ard that` applied to' the* other modulator4 Will' follovv" the cosine of the modulating' voltage.

4.` Means for addingtHe-otputs'of'tlemodllators in a linear manner.

In' previous systems item Glf) usuali-'yl' consists of a simpleY capacitor-resistor circuit', (2)" ol standard mixer typ'ef tuiles,l at leastA two tubes being necessary for? each modulator; and' (I4-) ol How' toi-` comply` with the requirement (3.1)? has.' presented' tlle greatest diiculty' inlV these previous systems; A prior known system. uses forv this purpose af-CR.l tube with a. sine= shapedl mask' o'rlV tlie screeri; By applying: a supersonic deflecting voltage toi the vertical deecting plates, a'vertical line trace is formed on the screen and this line is deflected across the mask by the modulating voltage. The desired sine and cosine relationship is obtained by utilising the light output from the screen.

' With this arrangement, however, the supersonic Y Jaw, The system must be duplicated in order to obtain the cosine relationship for the other modulator.

It will be readily appreciated that the above systems are complicated and unwieldy and re quire many` vacuum tubes besides the special apparatus for producing the sine and cosine laws. y It is therefore a further object of the present invention to provide an improved modulation system based on the principles outlined above in which the requirements (2) and (3) referred to are achieved by the used of a single valve.

The objects are achieved by providing, in accordance with the present invention, an electron beam discharge device having means for producing two grid controlled electron beams. said device functioning when supplied at the control grids with two carriers of equal frequency and phase quadrature relationship to produce two carriers modulated with a sine and cosine law with respect to modulating voltage, means whereby said carriers are added linearly in the anode system and an associated output circuit to give a phase modulated carrier of constant amplitude.

More specifically, a phase modulation system, in accordance with the present invention, comprises in combination an electron beam discharge device having a cathode or cathodes, means for deriving from said cathode or cathodes a pair of electron beams, means for independently modulating the intensity of said beams with II.F. energy in phase quadrature, an apertured plate spaced apart from said cathode or cathodes, each aperture having an associated anode electrode housed in screened compartments on the face of said plate remote from said cathode or cathodes, means for focussing said beams in spaced relationship on said apertured plate, means for linearly deflecting said beams across said apertures in dependence upon signal modulating potentials, and means for deriving output potentials from said anodes.

4 Still more specifically the phase modulating system, in accordance with the present invention, comprises, in combination, a thermionic valve having a cathode or cathodes, two independent control electrodes and an accelerating electrode with or without an additional focus electrode, the function of these electrodes being to produce two electron' beams of rectangular cross-section, means for modulating the intensity of said electron beams with HF. energy in phase quadrature relationship, electrostatic or electromagnetic deflecting means for producing an equal deflection of each beam at right angles to' its longer cross sectional dimension and linearly dependent on the modulating signal energy, a blanking plate with apertures cut in it in such a Way that in any position of the beams the portion of one beam Y, which is allowed to pass through will bear the relationship Y=iK sin X, where X is a distance measured from a predetermined position of this beam to its present position, and the portion of the other beam Z will bear the relationship Z=iK cos X, a series of anodes each placed directly behind an opening in the blanking plate and connected in two sets in such a way that the difference in the currents flowing to the sets from each beam follows the relationship Y=K sin X and Z=K cos X, means for deriving output energy from said anodes, and screening means for preventing interaction between the electron streams passing to adjacent anodes.

' For a more complete understanding of the nvention and the manner in which it is to be carried out attention is now directed to the following description in connection with the accompanying drawings in which:

Figure 1 is a schematic diagram showing the electrode arrangement in one particular embodi ment of an electron beam deiiection device employing the principles of the present invention;

Figure 2 represents a modification of the arrangement indicated by Figure 1 and illustrates the general layout of the electrode system within angevacuated envelope.

Figure 3 shows the physical relationship of the' electron beams focussed on the apertured plate.`

Figure 3A is a detail showing one method of connecting the anodes; and

Figure 4 shows the circuit arrangement used in conjunction with the device illustrated in Figure 1.

Referring to the gures in the drawings in which like parts are designated by similar reference numerals, a thermionic valve V is provided with two conventional heater cathode assemblies IA and IB each situated opposite the slit inthe respective one of its control grid electrodes 2A and 2B. A shield 3 is supported in any convenient manner between the two grid-cathode assemblies. Each grid is brought to a separate connection while the two cathodes and the shield are connected together. An accelerating anode 4 having two slits ASIV and ASZ is mounted scn that the slits ASI and ASE respectively register with the slits GSI and G52 (see Figure 2l) of the control grid electrodes 2A`and 2B and are at an appropriate distance therefrom.

Each side of the anode 4, which is parallel tov the long dimension of the anode slits ASI and ASZ is provided with an anode extension 4A. The extensions 4A are disposed in parallel planes and extend along Vthe longitudinal axis 4of the tube from that side of the anode which is remote from the cathodes IA, IB.`

A further extension 4B is attached to the anode lI intermediate the extensions 4A in a plane which isp-arallel to the planes of the extensions 4A and co-extensiveAwith the plane of the shield 3 toY partition the space path eiected by thel anode 4 into two paths which are symmetrically disposed with' respect to their associated anod slits. Y

Focussi'ng'ele'ctrodes of any convenient form, such as the single rods 5, are supported inthe present example in spaced relationshipwithin the space defined by the anode extensions 4A and 4B, one on either side of the electron paths leading from the individual cathodes IA and IB to the blanking plate 'I. The focussing electrodes 5 are connected together and brought out to a' separate connection.

ausgehe 52 electrode-arrangementathusfar described function: to: provide. two separate. beams of. electrons. Any convenient electron gun system which will provide two' spaced* electron beams of the type hereinafter described mayv ben employed Without affecting the scopeof the invention;

In the present examplev electrostaticl deflection is employed, two deflectingplatesSA- and BB'being supported in the space` between the focussing electrodesiand the blanking plate. 1. in any convenient. manner to. produce. simultaneous dei-lection of. the electron beams from the. cathod'es IA and IB. across the apertures A in the blanking plate. 1.. The denector plates Aand Barepr vided with .separate connections..

The blanking plate 'I is supported so that its length.isidisposedltransversely to. the longitudinal axis of; thetube and parallellto.thed'eection path otthespacedv electron beams.

The plate 'i is providedwith. sine shaped apertures, A: and is conductively connected to the. ac celerating. anode. ty by aV connection (not shown)v ofy low inductance. The anode d. and the plate 'I' are connected through a commonlead to a separate input connectionV (also not. shown) The suppressor assembly consists of a platelike... member `8 provided with. a number. of partitions which are so disposed as to form together withthe plate 8. a number of compartmentseach behinda sine shaped apertureA in the blanking plate 1.. The plate 8Y of the suppressor electrodel assembly isv conductively joined to ther cathodes IAa-nd. lB.by a connection of l`ow inductance (not shown) thewhole then beingplaced'lin communication witha separate terminal.

In the example indicated by Figure 1 the tube V5 hasplate-like members constituting anodes 9A and 91Ev which are insulatedly supported in individual compartments. of the suppressor electrode embracing the plate il.Y and are disposed so that one of' their broad surfaces faces the associa-ted aperture A in the blanking plate 1. The anodes 9A. and 9B are connected together in sets sothat all anodes marked .9A are connected together and placed in communication with one terminal and' all those marked SB' are connected Atogether and placed' in communication with another. terminal; The two sets of anodes 9A and 9B are, in the present invention,.connected' to terminals TA and TB respectively in thel tcp o f-.the tube Vwhilst all other electrodes are connectedthrough the` stem S to appropriate contact pins P locatedin the base B ofthe tube V..

From. the foregoing description itV will be seen that the electrodes which produce. the electron beams are the equivalent in electron optics of cylindrical lensesthat is,` they have a refractive eiect only in one plana The. combination of the grid andv anode slits. functions to form what is also referred to in electron optics as an immersion objective lens whichforrnsa cross-over pointih each beam near the plane of the anode slit. The combination of the focussing rods 5 within the.` structure of the accelerating, anode 4 forms what is known as a symmetrical electron lens.

When the correct ratio of voltages is applied to the electrodes, li. and5, electron. images L1 and Iz of the anode slits AS! and ASZ-V respectively, a-rervthrown onto-the blanking plate 'iA as. shown! in Figure 3. In thisgureit will-be seen that these images are at right` angles, to the base line L ofthe blanking. apertures A andthat they extend above and below the apertures due-tcithe diver..- gent or unfocussed-` nature of: the. beam in.` this dimension.. Figure 3r shows the. position. ot the beams whenA no;r deection. is` present. The; reiquirements: of. the. current density: distribution of. these electron images is that along the greater dimension. the density should. be constant and that across. the; shorter dimension the density should be symmetrically distributed. on. either side. of some centre line. If these two condtions hold fonanyworking valueof beam current, then it. can easilyl be proved that'the current entering a sine shaped aperture will be proportional to the' sine value corresponding to the position of the centre line of the beam. The actual width of the beam is not important, provided the above. conditions hold; Whenthe beam has passed the' blanking holes, its cross-sectional dimensions are nolonger important and itv maybe directed to an anode by the suppressor assembly which includes the plate 8.

The's-uppressor assembly being at cathode po-r tential prevents an interchange'of secondary elec-A trons between the blanking plate and the anodes and also between separate-anodes.

The output from atube constructed along the lines outlined above depends on the mutual conductances between the control grids and the anodes connected in a diiferential circuit. Thus it will be seen that for one beam this mutual conductance (and also the difference current) will be an exact sine functionV ofv the beam position (in-both positive and negative senses) As the two beams I1, I2 are maintained in fixed relationships they wiil be moved by the same amount for any deflection. It is obvious therefore that if their separation is made equivalent to` 99 degrees of the sine. shaped. apertures, then one beam can be said: to. follow a cosine. law while the other follows aA sine law. The deflectionsystem must, however, deflect bot-h beams in a strictly linear relationship with the voltage or current applied to it. In the case of electrostatic deflection, the desired effect may be achieved by the use of a balanced deflection system.

In the case of a small departure from linearity of deflection due, e. g., to the field gradient varying between the plates, the sine apertures can be suitably expanded towards the outer parts to compensate for this effect, If the angle of deflection is suflioiently wide to warrant it, the apertures A in the plate. 'l` could be arranged in an arc or a compression of the sine shapes could be used.

From the above it will bey seen that the two beam systems constitute two balanced modulators whose outputs are'directly added and which follow aA sine and cosine law over-a number of cycles, depending only on the dimensions of the tube, thus providing a simple means whereby the requirements 2 and 3 of thephase modulation system hereinbefore outlined may be carried out in a single tube.

A suitable circuit arrangement off a phase modulation system employing a tube of the type hereinbefore described is illustrated in Figure 4. Referring to this figure carrier frequency energy from any convenient source is applied in equal amplitude and in phase quadrature relationship to the grids 2A and 2B of the tube V through the input transformer Tl and the phase shifting network N.

rIhe phase shittingY network N functions to shift the phase cf the voltage fed to the input grids 45 degrees on either side of the carrier. Any conventional form ot phase-.shiftingnetwork such as will provide the desired phase relationship of the voltages fed to the grids may be employed. In the present example the well known resistance capacity phase shifting arrangement is illustrated.

The two electron beams produced within the tube are independently modulated with carrier frequency energy in phase quadrature relationship, thus dividing the carrier into two channels whose phase difference is 90 degrees.

Signal modulating potentials are applied in balanced relationship to the denecting plates 8A, 6B through the LF. coupling transformer T2.

Output potentials on the anodes 9A, 9B of the tube V are fed to a succeeding stage through the balanced coupling transformer T3.

The application of the signal modulating Potentialsrto the deflecting plates tA, 6B causes the two ELF. modulated electron beams, l1, l2 to be linearly deflected across the sine-shaped apertures A in the blanking plate l. spacing between the beams is equivalent to 90 degrees of the sine-shaped apertures, the output from one beam will follow a sine wave law and the output from the other a cosine law.

The outputs of the electron beams are added linearly in the output circuit associated with the anodes 9A, 9B to produce carrier frequency energy of constant amplitude modulated in phase in accordance with the amplitude of the signal energy applied to the deecting plates 5A, EB.

The extent of the phase deviation depends upon the number of apertures A in the plate 'l traversed by the deflection path of the beams.

In one practical embodiment of the invention the separation between the electron beams was made '7 mm. and the length of each sine-shaped aperture was 14 mm. A length of 25 mm. for

theV blanking plate thus produced a phase shift` in the nal output of :1:100 degrees.

The beams could, however, be focussed into a muchk narrower width and brought closer to-Y gether than the distance specified above, thus enabling a much greater phase deviation to be obtained. The use of a single second lens instead of two would facilitate this.

For example, for an available length of 3 cm. for the blanking plate suppose a beam spacing of` 2 mm. isused, the length representing'QO degrees of a sine aperture is also 2 mm. and the total phase displacement @gx 9o degrees= 1260 Thus, the available phase deviation would be ill radians. In the two applications previously mentioned, a multiplication of 30 orB in the mobile case or g? or 250 in the case of the broadcasting service would be necessary. At carrier frequencies of 100 megacycles, the Acrystal frequency would be over 20 megacycles for mobile and .4 mc. for broadcast service.

In the circuit of Figure 4, the requisite operating potentials may be applied to the various electrodes of the valve in conventional manner from any convenient potential supply source or sources (not shown) As the fixed 8. In one practical arrangement the following voltages were measured relative to the cathode;

2A and 2B -7 volts (4) +200 volts (5) +40 volts 6A and 6B 200 volts 80 volts 9A and 9B 250 volts Having now fully described and ascertained-my'- said invention and the manner in which it is to be performed I declare that what I claim is:

l. In a phase modulation system, means for producing a pair of'electron beams, means for independently varying the intensity of said beams in accordance with carrier frequency voltages in phase quadrature relationship, an apertured plate in the paths of said beams upon which said beams are caused to impinge in spaced relationship, a plurality of anodes equal in number Tto the number of apertures, each anode being housed in a separate compartment which communicates with a corresponding aperture and which is located on the face of said plate remote from the point of origin of said beams, means for deflecting said beams across said apertures in dependence upon modulating potentials, and means for deriving output potentials from the several anodes.

2. In a phase modulation system, means for producing two electron beams of substantially rectangular cross-section, means for independently Varying the intensity of said beams in yac cordance with carrier frequency voltages in phase quadrature relationship, means for producing an equal deflection of each beam at right angles to its longer cross-sectional dimension in dependence upon modulating energy, an aperturedv plate in the paths of said beams, said plate having its apertures so disposed that in any instantaneous v position of said beams the portion Y of one beam which is allowed to pass through said aperturesis equal to iK sin X, where K is a constant and X is a distance measured from a predetermined position of said one beam to its'position at that instant, and the portion Z of the other beam which is allowed to pass through said apertures is equal to iK cos X, a plurality of anodes one of which is placed directly behind each aperture in said plate, means connecting said anodes in two groups in such manner that the diierence in currents flowing to said groups from each of said beams follows the relationship Y=K sin X and Z=K cos X, means for preventing interaction between the electron streams flowing to adjacent anodes, and` means including in part said connecting means for deriving output potentials from said anodes. Y

3. A phase modulation system as defined in claim 2, wherein the means for producing the two electron beams includes two control grids with slits therein and an adjacent anodel with similar slits, the slits having such outline as to impart to said beams a cross-section which is long and narrow.

4. A phase modulation system as defmedf in claim 2, wherein the apertures in said plate'ar'e of substantially sinusoidal outline and wherein said connecting means connects said anodes `al'- ternately in two groups.

5. A phase modulation system as defined in claim 2, wherein the apertures in said plate are of substantially sinusoidal outline, wherein said connecting means connects said anodes alternately in two groups, and wherein said means for deriving output potentials includesa balanced coupling transformer from which is derived resultant carrier frequency energy of constant amplitude modulated in phase in accordance with the amplitude of the modulating energy.

6. In a phase modulation system for modulation of a carrier frequency voltage, means for producing two electron beams, means for independently varying the intensity of said beams in accordance with voltages of said carrier frequency in phase quadrature relationship, an apertured plate in the paths of said beams, the apertures therein being of substantially sinusoi dal outline, and means for deecting said beams across said apertures in dependence upon modu'- lating potentials.

7. In a phase modulation system for modulation of a carrier frequency voltage, means for producing two electron beams, means for independently varying the intensity of said beams in accordance with voltages of said carrier frequency in phase quadrature relationship, an apertured plate in the paths of said beams, the apertures therein being of substantially sinusoidal outline, means for deflecting said beams across said apertures in dependence upon modulating potentials, and an electron-receiving electrode system in the paths of said beams and behind said plate, comprising a plurality of electrodes connected in two groups, the arrangement being such that the output from one beam conforms to a sine law and the output from the other beam conforms to a cosine law.

8. A phase modulator electron beam discharge device comprising means for producing a pair of electron beams, a control electrode in the path of each beam for independently varying the intensity of said beams in accordance with carrier frequency voltages in phase quadrature relationship, an apertured plate in the paths of said beams upon which said beams are caused to impinge in spaced relationship during operation of said device, a plurality of ancdes equal in number to the number of apertures, each anode being housed in a separate compartment which communicates with a corresponding aperture and which is located on the face of said plate remote from the point of origin of said beams, deflecting electrode means for deiiecting said beams across said apertures in dependence upon modulating potentials, and terminal means for deriving output potentials from the several anodes.

9. A phase modulator electron beam discharge device comprising means for producing two electron beams, a control electrode in the path of each beam for independently varying the intensity of said beams in accordance with carrier frequency voltages in phase quadrature relationship, an apertured plate in the paths of said beams, the apertures therein being of substantially sinusoidal outline, and deflecting electrode means for deecting said beams across said apertures in dependence upon modulating potentials.

EDWARD GOODMAN HOPKINS.

REFERENCES CITED The following references are of record in the Ele of this patent:

UNITED STATES PATENTS Number Name Date 2,201,323 Shelby May 21, 1940 2,294,209 Roder A,ug. 25, 1942 2,337,272 Roberts Dec. 21, 1943 

